Properties And Energy Storage Mechanisms Of Alkali Metal Pre-treated Spinel Mn₃O₄ As Cathode For Aqueous Magnesium-ion Battery

Aqueous ion batteries have excellent application prospects in large-scale energy storage devices due to their lower cost and higher safety.However,due to the limitation of water decomposition voltage,the energy density of aqueous ion batteries is poor.In order to improve its energy density,the multivalent-ion battery system can be used to significantly improve the energy density and power density of an energy storage systems,since the carriers carry two or more charges.However,just because of the multivalent-ion’s nature itself,the coulombic interaction is strong between them and the framework of the cathode material.As a consequence,it leads to the disintegration and capacity loss of the cathode material during the electrochemical process due to the incomplete extraction of the multivalent ions.In this thesis,on the basis of an aqueous magnesium-ion battery,the spinel Mn₃O₄ is used as the precursor of the cathode material.The crystal structure of the manganese oxide cathode material is controlled by pre-treatment to improve the diffusion rate of magnesium-ion and reversible electrochemical reaction.This can facilitate to build up an aqueous magnesium-ion battery with high capacity and high cycle stability.The main research contents and results of this thesis are as follows:（1）Electrochemical performance and storage mechanism of Na⁺-pre-treated spinel Mn₃O₄ as aqueous Mg²⁺-ion battery cathode material.The spinel Mn₃O₄ directly prepared by using a simple air oxidation and precipitation method.The prepared spinel Mn₃O₄ is made into electrode sheets,which are pre-treated in sodium sulfate（Na₂SO₄）solution.As a result,the Mn₃O₄-A with amorphous ion channel and grain refinement are obtained.Electrochemical performance of Mn₃O₄-A reveals an optimal Mg²⁺storage performance,and an excellent rate performance and cycle stability.Through analyzing electrochemical kinetics,ex-situ XRD and ex-situ XPS,the electrochemical storage mechanism has been further explored:Mg²⁺ions insert and extract in the amorphous ion diffusion channels;there is a（sub-）surface pseudocapacitance reaction between the Mn（II）/（III）during（dis）charging processes.（2）Effects of alkali-ion（Li⁺/Na⁺/K⁺）pre-treatment on spinel Mn₃O₄electrochemical properties and kinetics of Mg²⁺.The preparation method of air oxidation to generate Mn₃O₄ is optimized,and the highly crystalline Mn₃O₄nanoparticles are obtained with more uniform morphology and smaller grain size.Mn₃O₄-M（M=Li,Na,K）electrode materials with different crystallinity were obtained by using electrochemical pretreatment in Li⁺/Na⁺/K⁺electrolytes as precursors.Since Li⁺has the smallest ionic radius,it is easier to considerably intercalate into the spinel structure during the pretreatment process.As a result,it destroys the crystallinity of the whole electrode material.However,K⁺has little damage to the spinel structure due to its largest ionic radius.Combined with electrochemical performance and electrochemical kinetics,the effects of amorphous structure and grains in the spinel structure after pretreatment on the performance and mechanism of Mg²⁺are investigated.The amorphous structure greatly increases the diffusion rate of Mg²⁺;concurrently,the active sites are generally located on the grain surfaces.The more amorphous structures,the less active sites,which lead to the decrease of the capacity.Therefore,based on Mn₃O₄-M（M=Li,Na,K）,a package of excellent Mg²⁺storage performance must address a trade-off to balance the amorphous and crystalline structures.This provides a reference for designing aqueous high-performance and high-stability Mg-ion battery cathode materials.